#! /bin/bash

a=10
b=20

echo "a的值是$a b的值是$b"
echo "$a+$b=$((a+b))" 
val=`expr $a + $b`
echo "$a+$b=$val"

echo "a的值是${a} b的值是${b} 和是`expr $a + $b`"
echo "a的值是${a} b的值是${b} 和是$((a+b))"
echo "a的值是${a} b的值是${b} `expr $a + $b`"